It has been shown that bone zinc can become available to maintain zinc nutrition during extended periods of zinc-free feeding. Periods of normal growth, freedom from deficiency symptoms and survival time were related to levels of bone zinc which were in turn related to the amount of zinc administered during a zinc loading period in the first ten days after weaning. Bone zinc decreased from relatively high levels during the period of observation and reached deficient levels when symptoms became evident. Germfree rats appear to require less zinc than non-germfree rats; female rats apparently require less zinc than males especially in a germfree state. Although the germfree state did not markedly delay onset of vitamin A deficiency, it did result in somewhat less severe symptoms and increased survival time. A germfree state apparently increases food efficiency for rats with limited food intake but not for ad-lib fed rats. By dietary means we have produced rats exhibiting severe symptoms of tryptophan deficiency, including marked hypersensitivity, tremors, seizures and decreased levels of 5-hydroxytryptamine in brain and other organs. Hopefully these will be suitable models to attempt to correlate the abnormal behavior with alterations in the biochemistry of neuronal systems.